Optical transmission system and fiber type determination method

ABSTRACT

An optical transmission system includes a first optical node, a second optical node, and an optical fiber provided between the first optical node and the second optical node. The optical transmission system further includes: a signal generator provided in the first optical node and configured to generate an optical signal including a plurality of wavelength channels and an empty channel; an optical transmission circuit provided in the first optical node and configured to output the optical signal to the optical fiber; an optical channel monitor provided in the second optical node and configured to measure reception power of each channel in the optical signal received through the optical fiber; and a processor configured to determine a type of the optical fiber based on the reception power of the empty channel, the reception power being measured by the optical channel monitor.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2022-054247, filed on Mar. 29,2022, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to an optical transmissionsystem and a method for determining a type of an optical fiber used inthe optical transmission system.

BACKGROUND

A type of an optical fiber to be used in an optical communicationnetwork is determined in accordance with uses, costs, or the like. Inrecent years, a single-mode optical fiber (SMF), a dispersion shiftedsingle-mode optical fiber (DSF), a non-zero dispersion shiftedsingle-mode optical fiber (NZ-DSF), or the like has been used in anoptical communication network.

An SMF is an optical fiber of which the core diameter is decreased toallow propagation in only one mode. A general-purpose SMF has azero-dispersion wavelength in a 1310-nm band and is therefore frequentlyused in a backbone network for which high-quality and stablecommunications are required with low transmission loss. A DSF has azero-dispersion wavelength in a 1550-nm band where transmission loss islow, and is therefore frequently used for long-distance transmission. AnNZ-DSF has a zero-dispersion wavelength that is slightly shifted fromthe 1550-nm band. For example, the NZ-DSF has a zero-dispersionwavelength at approximately 1500 nm. Since this configuration suppressesa non-linear effect in the 1550-nm band, the NZ-DSF is suitable forwavelength division multiplexing transmission and is frequently used forultra-high-speed long-distance transmission.

An optical transceiver and an optical amplifier to be provided on eachoptical node in an optical transmission system are required to bedesigned in accordance with a type of an optical fiber. Therefore, acommunication carrier checks a type of an optical fiber laid on eachspan.

FIG. 1 illustrates an exemplary method for detecting a type of anoptical fiber laid between optical nodes. For example, in detecting atype of an optical fiber F1 laid between optical nodes N1 and N2, alight source (LD) is connected to one of the optical nodes (N1), and adispersion measuring instrument is connected to the other optical node(N2). Light output from the light source propagates from the opticalnode N1 to the optical node N2 through the optical fiber F1. The type ofthe optical fiber F1 is determined in such a manner that the dispersionof the received light is measured using the dispersion measuringinstrument.

A method for estimating a type of an optical fiber, based on reflectedlight power from an optical transmission line has also been proposed(for example, Japanese Laid-open Patent Publication 2007-173969).

Such a conventional method needs much effort for determining a type ofan optical fiber. According to, for example, the method illustrated inFIG. 1 , operators need to be positioned at two ends of a target opticalfiber. Each span is several kilometers to 100 km. In addition, alarge-scale optical communication network includes a large number ofoptical nodes. Therefore, considerable effort has been spent indetermining a type of an optical fiber for each span in the opticalcommunication network.

SUMMARY

According to an aspect of the embodiments, an optical transmissionsystem includes a first optical node, a second optical node, and anoptical fiber provided between the first optical node and the secondoptical node. The optical transmission system further includes: a signalgenerator provided in the first optical node and configured to generatean optical signal including a plurality of wavelength channels and anempty channel; an optical transmission circuit provided in the firstoptical node and configured to output the optical signal to the opticalfiber; an optical channel monitor provided in the second optical nodeand configured to measure reception power of each channel in the opticalsignal received through the optical fiber; and a processor configured todetermine a type of the optical fiber based on the reception power ofthe empty channel, the reception power being measured by the opticalchannel monitor.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an exemplary method for detecting a type of anoptical fiber laid between optical nodes;

FIG. 2 illustrates an exemplary optical transmission system according toan embodiment of the present invention;

FIG. 3 illustrates an exemplary configuration of an optical nodeaccording to an embodiment of the present invention;

FIG. 4 illustrates an exemplary OTDR;

FIG. 5 illustrates an exemplary result of measurement by the OTDR;

FIGS. 6A and 6B each illustrate a relationship between a propagationtime measured by the OTDR and a wavelength;

FIG. 7 illustrates a relationship among a propagation time, awavelength, and a type of an optical fiber;

FIG. 8 illustrates wavelength dispersion of an SMF, a DSF, and anNZ-DSF;

FIGS. 9A-9D illustrate an exemplary method for determining a type of anoptical fiber, using a pseudo WDM signal;

FIG. 10 illustrates an exemplary method for calculating crosstalk;

FIG. 11 is a flowchart illustrating an exemplary method for determininga type of an optical fiber;

FIG. 12 illustrates another example of an optical node realized by aROADM;

FIG. 13 illustrates another exemplary optical transmission systemaccording to an embodiment of the present invention;

FIG. 14 illustrates an exemplary optical node operable as an opticalrelay station;

FIG. 15 illustrates an exemplary optical transmission system in whichtwo types of optical fibers are mixed in one span;

FIGS. 16A and 16B illustrate an exemplary method for identifying the twotypes of the optical fibers based on crosstalk;

FIG. 17 illustrates an exemplary configuration of an optical node usedin the optical transmission system illustrated in FIGS. 16A and 16B;

FIGS. 18A-18D illustrate an exemplary method for controllingtransmission power of a WDM signal in accordance with a type of anoptical fiber;

FIG. 19 illustrates an exemplary sequence of cooperative operation by aset of optical nodes;

FIG. 20 illustrates an exemplary arrangement of an OSC;

FIG. 21 illustrates an exemplary optical node including an OSCprocessor;

FIG. 22 is a flowchart illustrating an exemplary method for identifyingan SMF, a DSF, and an NZ-DSF, using a pseudo WDM signal; and

FIGS. 23A-23C illustrate another exemplary method for determining a typeof a fiber based on crosstalk.

DESCRIPTION OF EMBODIMENTS

FIG. 2 illustrates an exemplary optical transmission system according toan embodiment of the present invention. An optical transmission system100 according to an embodiment of the present invention includes aplurality of optical nodes 1 (1 a to 1 n). Each optical node 1 includesan optical transmission device. The optical transmission system 100 is aWDM transmission system for transmitting a wavelength divisionmultiplexing signal. Therefore, the optical transmission device of eachoptical node 1 is, for example, a reconfigurable optical add-dropmultiplexer (ROADM). In the following description, the wavelengthdivision multiplexing signal may be referred to as a “WDM signal”.

The optical nodes 1 are connected to each other with optical fibers 2.Optical signals are transmitted bidirectionally between two opticalnodes. For example, an optical signal is transmitted from the opticalnode 1 a to the optical node 1 b through the optical fiber 2 x, and anoptical signal is transmitted from the optical node 1 b to the opticalnode 1 a through the optical fiber 2 y.

FIG. 3 illustrates an exemplary configuration of one of the opticalnodes 1 according to an embodiment of the present invention. The opticalnode 1 includes a wavelength selective switch (WSS) 11, an opticalamplifier circuit 12, an optical time domain reflectometer (OTDR) 13, anoptical switch 14, an optical channel monitor (OCM) 15, an opticalswitch 16, an amplified spontaneous emission (ASE) light source 17, anda controller 18. It should be noted that the optical node 1 may includeany elements, circuits, and functions not illustrated in FIG. 3 . Anoptical fiber TX and an optical fiber RX are connected to the opticalnode 1.

The WSS 11 processes a WDM signal including a plurality of wavelengthchannels. For example, the WSS 11 is capable of inserting an opticalsignal into an empty channel of the WDM signal, in accordance with aninstruction from the controller 18. The WSS 11 is also capable ofextracting an optical signal from an instructed one of the wavelengthchannels by the controller 18. The optical amplifier circuit 12includes, for example, an erbium doped fiber amplifier (EDFA) and iscapable of amplifying a WDM signal. In this example, the opticalamplifier circuit 12 amplifies a WDM signal generated by the WSS 11 andalso amplifies a WDM signal received through the optical fiber RX. Theoptical amplifier circuit 12 has a gain to be controlled by thecontroller 18. The optical amplifier circuit 12 is an example of atransmission light circuit configured to transmit a pseudo WDM signal tobe described later.

The OTDR 13 is an optical measuring device configured to detect adiscontinuity on an optical fiber. Specifically, the OTDR 13 emits anoptical pulse to the optical fiber and detects reflected light from theoptical fiber. The OTDR 13 is capable of detecting a position of thediscontinuity on the optical fiber, based on power and a timing of thereflected light. It should be noted that the discontinuity on theoptical fiber includes a fault location and an end. The discontinuity onthe optical fiber also includes a connection between optical fibers. Theoptical switch 14 configures an optical path between the OTDR 13 and anoptical fiber. For example, in measuring the optical fiber TX, theoptical switch 14 configures an optical path between the OTDR 13 and anoptical coupler C1. Moreover, in measuring the optical fiber RX, theoptical switch 14 configures an optical path between the OTDR 13 and anoptical coupler C2.

The optical channel monitor 15 is capable of measuring power of eachwavelength channel in a WDM signal. The optical switch 16 configures anoptical path in accordance with a WDM signal to be measured by theoptical channel monitor 15. For example, in monitoring a WDM signalreceived through the optical fiber RX, the optical switch 16 configuresan optical path between the optical channel monitor 15 and an opticalcoupler C3. In monitoring a reception WDM signal amplified by theoptical amplifier circuit 12, the optical switch 16 configures anoptical path between the optical channel monitor 15 and an opticalcoupler C4. In monitoring a transmission WDM signal generated by the WSS11, the optical switch 16 configures an optical path between the opticalchannel monitor 15 and an optical coupler C5. In monitoring atransmission WDM signal amplified by the optical amplifier circuit 12,the optical switch 16 configures an optical path between the opticalchannel monitor 15 and an optical coupler C6.

The ASE light source 17 generates ASE light. It is assumed herein thatthe ASE light source 17 is capable of generating high-power andbroadband ASE light. The ASE light generated by the ASE light source 17is guided to the WSS 11. The WSS 11 generates a pseudo WDM signal (to bedescribed later), using this ASE light. It should be noted that acombination of the ASE light source 17 and the WSS 11 is an example of asignal generator configured to generate a pseudo WDM signal.

The controller 18 includes a determination unit 18 a and controlsoperation of the optical node 1. For example, the controller 18instructs the OTDR 13 to measure an optical fiber. In this case, thedetermination unit 18 a determines a type of the optical fiber, based ona result of the measurement by the OTDR 13. Moreover, the controller 18instructs the WSS 11 to generate a pseudo WDM signal. In addition, whenthe optical node 1 receives a pseudo WDM signal transmitted from anadjacent node, the determination unit 18 a may determine a type of anoptical fiber, using the received pseudo WDM signal.

It should be noted that the controller 18 is realized by, for example, aCPU including a processor and a memory. In this case, the processorexecutes a fiber type determination program stored in the memory,thereby determining a type of an optical fiber connected to the opticalnode 1. However, a part of functions of the controller 18 may berealized by a hardware circuit.

FIG. 4 illustrates an exemplary OTDR 13. In this example, the OTDR 13includes a pulse generator 13 a, a light source circuit 13 b, a photodetector circuit 13 c, and a signal processor 13 d. The OTDR 13 measuresan optical fiber in accordance with an instruction given by thecontroller 18.

The pulse generator 13 a generates a pulse signal in accordance with ageneration instruction given by the signal processor 13 d. The lightsource circuit 13 b generates an optical pulse in synchronization withthe pulse signal generated by the pulse generator 13 a. This opticalpulse is emitted to a measurement target optical fiber. The light sourcecircuit 13 b includes two LD light sources (λ1, λ2). That is, the lightsource circuit 13 b is capable of generating an optical pulse with awavelength λ1 and an optical pulse with a wavelength λ2. Although notparticularly limited, λ1 and λ2 are respectively 1310 nm and 1650 nm,for example.

When the OTDR 13 emits light to an optical fiber, Rayleigh scatteringoccurs, and part of the backscattering light returns to the OTDR 13. Inaddition, when Fresnel reflection occurs at a discontinuity on anoptical fiber, the reflected light also returns to the OTDR 13.

The photo detector circuit 13 c converts the reflected light(backscattering and Fresnel reflection) into electric signals. Thesignal processor 13 d measures a state of an optical fiber, based on anoutput signal from the photo detector circuit 13 c. That is, the signalprocessor 13 d measures a state of an optical fiber, based on reflectedlight of the optical pulse emitted to the optical fiber.

FIG. 5 illustrates an exemplary result of measurement by the OTDR 13. Inthe graph, the horizontal axis represents an elapsed time fromtransmission of an optical pulse from the OTDR 13. The vertical axisrepresents power of reflected light from an optical fiber. The reflectedlight is detected by the photo detector circuit 13 c.

The intensity of reflected light caused by Rayleigh scattering dependson a distance from the OTDR 13. In other words, reflected light from aposition closer to the OTDR 13 is stronger, whereas reflected light froma position farther from the OTDR 13 is weaker. Therefore, power ofreflected light to be detected by the photo detector circuit 13 cbecomes gradually smaller as an elapsed time from transmission of anoptical pulse from the OTDR 13 is longer. However, the photo detectorcircuit 13 c also detects reflected light caused by Fresnel reflectionoccurring at a discontinuity on an optical fiber. At the timing when thereflected light from the discontinuity is detected, the power of thereflected light rapidly changes. Therefore, measuring the time from thetransmission of the optical pulse from the OTDR 13 to the timing atwhich the power of the reflected light rapidly changes enablescalculation of a transmission distance from the OTDR 13 to thediscontinuity on the optical fiber. It should be noted that thediscontinuity on the optical fiber includes an end of the optical fiber(an adjacent node in FIG. 4 ).

As described above, the OTDR 13 is capable of detecting thediscontinuity on the optical fiber by measuring a propagation time fromthe transmission of the optical pulse to the reflected light receptiontiming. This propagation time depends on a wavelength of the opticalpulse and a type of the optical fiber.

FIGS. 6A and 6B each illustrate a relationship between a propagationtime measured by the OTDR 13 and a wavelength. FIG. 6A illustrates acase where a measurement target optical fiber is an SMF, and FIG. 6Billustrates a case where a measurement target optical fiber is a DSF oran NZ-DSF. In this example, a measurement is made on a propagation timeuntil an optical pulse transmitted from the OTDR 13 is reflected at aspecified discontinuity point K and returns to the OTDR 13. Thediscontinuity point K may be an end of the measurement target opticalfiber (i.e., an adjacent node).

In the case where the measurement target optical fiber is the SMF, lightwith a wavelength of 1310 nm (hereinafter, referred to as λ1 light) hasa propagation time of t1, and light with a wavelength of 1650 nm(hereinafter, referred to as λ2 light) has a propagation time of t2. Inthis example, the propagation time of the λ2 light is longer than thepropagation time of the λ1 light. In addition, a difference ΔTs betweenthe two propagation times is relatively large.

In the case where the measurement target optical fiber is the DSF or theNZ-DSF, the λ1 light has a propagation time of t3 and the λ2 light has apropagation time of t4. In this example, the propagation time of the λ2light is shorter than the propagation time of the λ1 light. In addition,a difference ΔTd between the two propagation times is relatively small.

FIG. 7 illustrates a relationship among a propagation time, awavelength, and a type of an optical fiber. The horizontal axisrepresents a wavelength of an optical pulse transmitted from the OTDR13. The vertical axis represents a propagation time until the opticalpulse transmitted from the OTDR 13 returns to the OTDR 13. However, thispropagation time indicates a relative value with respect to apropagation time on condition that a wavelength is 1550 nm. In addition,a propagation distance is 100 km. A solid line represents a case where ameasurement target optical fiber is an SMF. A broken line represents acase where a measurement target optical fiber is a DSF. A chain linerepresents a case where a measurement target optical fiber is an NZ-DSF.

The OTDR 13 measures propagation times, using the λ1 light and the λ2light as described above. In this example, λ1 is 1310 nm and λ2 is 1650nm.

In the case where the measurement target optical fiber is the SMF, thegradient of a curve indicating a propagation time with respect to awavelength is large. Therefore, as indicated by circular marks, adifference between the propagation time relative to λ1 and thepropagation time relative to λ2 is large. Specifically, the differencebetween the propagation times is 3.82 nm/km. In this case, thedifference between the propagation times is expressed by approximately7.6×D [nm] in which D [km] represents a distance from the OTDR 13 to areflection point (i.e., a discontinuity on an optical fiber or anadjacent node).

In the case where the measurement target optical fiber is the DSF or theNZ-DSF, the gradient of a curve indicating a propagation time withrespect to a wavelength is small. Therefore, as indicated by triangularmarks or rectangular marks, a difference between the propagation timerelative to λ1 and the propagation time relative to λ2 is small.Specifically, for example, the difference between the propagation timesabout the DSF is −0.85 nm/km. In this case, the difference between thepropagation times is expressed by approximately 1.9×D [nm].

Therefore, when setting a specified threshold value, it is possible toidentify the SMF and the DSF-based fibers (the DSF and the NZ-DSF). Forexample, in allowing a variation of 50 percent, a threshold value rangefor determining whether the measurement target optical fiber is the SMFis 3.8×D to 11.4×D [nm]. Specifically, it is determined that themeasurement target optical fiber is the SMF when the difference betweenthe propagation times falls within the threshold value range, and it isdetermined that the measurement target optical fiber is one of theDSF-based fibers when the difference between the propagation timesdeviates from the threshold value range. Alternatively, it may bedetermined that the measurement target optical fiber is the SMF when thedifference between the propagation times falls within the thresholdvalue range, and it may be determined that the measurement targetoptical fiber is one of the DSF-based fibers when the difference betweenthe propagation times is smaller than a lower limit value of thethreshold value range.

In the examples illustrated in FIGS. 6A, 6B, and 7 , the two wavelengthsused by the OTDR 13 are 1310 nm and 1650 nm; however, an embodiment ofthe present invention is not limited to this configuration. However, ifthe difference between the two wavelengths is excessively small, thedifference between the propagation times becomes small, which maydecrease accuracy in determining a type of an optical fiber. It istherefore preferable to increase the difference between the twowavelengths within a range in which an optical loss is small. It is alsopreferable to use an inexpensive light source in consideration ofmanufacturing costs of the optical node 1.

As described above, the fiber type determination method according to anembodiment of the present invention is capable of identifying the SMFand the DSF-based fibers. Next, a description will be given of a methodfor identifying the DSF and the NZ-DSF.

FIG. 8 illustrates wavelength dispersion of the SMF, the DSF, and theNZ-DSF. In this example, the dispersion of the DSF at 1550 nm is zero.It should be noted that the NZ-DSF has a zero-dispersion wavelength ofapproximately 1500 nm.

A non-linear effect in a case where an optical fiber receives multiplelight waves with different wavelengths depends on an amount ofdispersion. For example, an amount of crosstalk by four-wave mixing(FWM) increases in a zero-dispersion wavelength. Therefore, an amount ofcrosstalk occurring at 1550 nm in a case where the DSF receives multiplelight waves is larger than an amount of crosstalk occurring at 1550 nmin a case where the NZ-DSF receives the multiple light waves. Hence, thefiber type determination method according to an embodiment of thepresent invention identifies the DSF and the NZ-DSF, using thischaracteristic.

FIGS. 9A-9D illustrate an exemplary method for determining a type of anoptical fiber, using a pseudo WDM signal. The pseudo WDM signal isgenerated by the ASE light source 17 and the WSS 11, based on aninstruction from the controller 18, in the optical node 1 illustrated inFIG. 3 .

FIG. 9A illustrates ASE light generated by the ASE light source 17. TheASE light has large power over a wide band. The ASE light is guided to aspecified input port of the WSS 11.

FIG. 9B illustrates an exemplary pseudo WDM signal output from the WSS11. The pseudo WDM signal is achieved by a plurality of wavelengthchannels CH1 to CH10 and CH12 to CH21 arranged at specified wavelengthspacing. Although not particularly limited, the specified wavelengthspacing (frequency spacing) is, for example, 50 GHz. Alternatively,these wavelength channels may be arranged on a frequency grid defined byITU-T. It is assumed that each wavelength channel has transmission powerequal to or more than a specified level. However, no wavelength channelis configured for the zero-dispersion wavelength of the DSF. In otherwords, an empty channel is configured for the zero-dispersion wavelengthof the DSF. The empty channel has transmission power set to be lowerthan transmission power of at least a wavelength channel fortransmitting a signal (i.e., a signal channel). In this example, thezero-dispersion wavelength of the DSF is 1550 nm. As illustrated in FIG.9B, therefore, no wavelength channel is configured at 1550 nm. The emptychannel is preferably configured at the zero-dispersion wavelength ofthe DSF, but may be configured near the zero-dispersion wavelength ofthe DSF.

The controller 18 gives to the WSS 11 an instruction to allowtransmission of light through the wavelength channels CH1 to CH10 andCH12 to CH21 and to block light with other wavelengths. The WSS 11processes the ASE light in accordance with this instruction. The WSS 11thus generates a pseudo WDM signal that transmits the wavelengthchannels CH1 to CH10 and CH12 to CH21 and in which the optical power at1550 nm is substantially zero. In the following description, awavelength for which a wavelength channel is not configured and in whichthe optical power is controlled to be substantially zero may be referredto as an “empty channel wavelength”.

The optical node 1 transmits the generated pseudo WDM signal to theadjacent node. The adjacent node receives the pseudo WDM signal throughthe optical fiber. In the adjacent node, the received pseudo WDM signalis guided to the optical channel monitor 15. It is assumed that theadjacent node has the same configurations as those of the optical node 1illustrated in FIG. 3 .

When the pseudo WDM signal propagates through the optical fiber,crosstalk by four-wave mixing occurs. An amount of the crosstalk by thefour-wave mixing depends on the wavelength dispersion of the opticalfiber. Specifically, large crosstalk occurs in the zero-dispersionwavelength of the optical fiber. The zero-dispersion wavelength of theDSF is 1550 nm. Therefore, in a case where the optical fiber throughwhich the pseudo WDM signal propagates is the DSF, large optical poweris detected at 1550 nm as illustrated in FIG. 9C. In contrast to this,the zero-dispersion wavelength of the NZ-DSF is shifted from 1550 nm.Therefore, in a case where the optical fiber through which the pseudoWDM signal propagates is the NZ-DSF, small optical power is detected at1550 nm as illustrated in FIG. 9D. Although not particularlyillustrated, also in a case where the optical fiber through which thepseudo WDM signal propagates is the SMF, small optical power is detectedat 1550 nm.

As described above, when the optical node 1 transmits the pseudo WDMsignal to the adjacent node, the adjacent node is capable of determiningwhether the optical fiber between the optical node 1 and the adjacentnode is the DSF, based on the optical power detected in the emptychannel wavelength. Specifically, it is determined that the opticalfiber between the optical node 1 and the adjacent node is the DSF whenthe optical power detected in the empty channel wavelength is largerthan a specified threshold value. On the other hand, it is determinedthat the optical fiber between the optical node 1 and the adjacent nodeis a fiber different from the DSF (the NZ-DSF or the SMF) when theoptical power detected in the empty channel wavelength is smaller thanthe specified threshold value.

It should be noted that the fiber type determination method according toan embodiment of the present invention uses large crosstalk occurring inthe zero-dispersion wavelength of the DSF, in order to identify the DSFand the NZ-DSF. If it is assumed herein that a wavelength channel isconfigured for the zero-dispersion wavelength of the DSF, it becomesdifficult to detect crosstalk occurring by four-wave mixing. Therefore,the use of the pseudo WDM signal in which the optical power in thezero-dispersion wavelength of the DSF is substantially zero enablesaccurate detection of crosstalk occurring in the zero-dispersionwavelength of the DSF.

Alternatively, a pseudo WDM signal in which the optical power in thezero-dispersion wavelength of the NZ-DSF is substantially zero may beused in place of the pseudo WDM signal in which the optical power in thezero-dispersion wavelength of the DSF is substantially zero. This caseenables a determination as to whether the optical fiber between theoptical node 1 and the adjacent node is the NZ-DSF or a fiber differentfrom the NZ-DSF (the DSF or the SMF).

FIG. 10 illustrates an exemplary method for calculating crosstalk. Inthis example, the optical node 1 a transmits the pseudo WDM signalillustrated in FIG. 9B to the optical node 1 b. In the optical node 1 b,the optical channel monitor 15 measures a ratio between average power ofthe wavelength channels (e.g., CH1 to CH10, CH12 to CH21) and power ofthe empty channel wavelength in the pseudo WDM signal. The opticalcomponent of the empty channel wavelength corresponds to noise for thewavelength channels CH1 to CH10 and CH12 to CH21. In the followingdescription, therefore, the foregoing ratio may be referred to as“OSNR_OCM”.

The determination unit 18 a calculates crosstalk by four-wave mixingbased on the OSNR_OCM obtained by the measurement. Specifically, thecrosstalk FWMXT is expressed by formula (1). It should be noted thatformula (1) is calculation of antilogarithm.

$\begin{matrix}{\frac{1}{FWMXT} = {\frac{1}{{OSNR}_{-}{OCM}} - \frac{1}{{OSNR}_{-}{AMP}}}} & (1)\end{matrix}$

In formula (1), OSNR_AMP represents an optical signal-to-noise ratio ofthe optical amplifier circuit 12 that serves as a preamplifier in areception node. The OSNR_AMP is calculated by formula (2). It should benoted that formula (2) is calculated in dB.

OSNR_AMP=P_in−NF(P_in)−(−57.938 dBm)   (2)

In formula (2), P_in represents input power in the reception node. Forexample, light that arrives at the optical node through the opticalfiber RX illustrated in FIG. 3 is guided by the optical coupler C3 tothe optical channel monitor 15. The optical channel monitor 15 istherefore capable of measuring the input power. Also in formula (2), NFrepresents a noise figure of the optical amplifier circuit 12 and is afunction of the input power P_in. It is assumed herein that thecharacteristics of the NF are known. Therefore, the noise figure NF iscalculated by measuring the input power P_in.

The crosstalk FWMXT is calculated by formula (1) and formula (2) above.The determination unit 18 a determines that the optical fiber betweenthe optical nodes 1 a and 1 b is the DSF when the crosstalk FWMXT islarger than a specified threshold value.

A transmission distance between the optical nodes 1 a and 1 b is known.It is assumed that the relationship between the fiber input power andthe crosstalk is found in advance as illustrated in FIG. 10 for thetransmission distance. The fiber input power corresponds to transmissionpower in a transmission node (the optical node la in FIG. 10 ).

For example, when the fiber input power is 4 dBm, the crosstalk of theDSF is −26 dBm, and the crosstalk of the optical fiber different fromthe DSF is −33 dBm. An intermediate value between the two values isdefined as a threshold value. That is, the threshold value is −29.5 dBm.In this case, it is determined that the optical fiber between theoptical nodes 1 a and 1 b is the DSF when the crosstalk is larger than−29.5 dBm. On the other hand, it is determined that the optical fiberbetween the optical nodes 1 a and 1 b is the optical fiber differentfrom the DSF when the crosstalk is smaller than −29.5 dBm.

The zero-dispersion wavelength of the DSF may have variations. For thisreason, when the empty channel wavelength is configured at 1550 nm, theempty channel in the pseudo WDM signal may be shifted from thezero-dispersion wavelength of the DSF. When the empty channel in thepseudo WDM signal is shifted from the zero-dispersion wavelength of theDSF, the reception node is incapable of accurately detecting thecrosstalk. Hence, the optical node 1 transmits the pseudo WDM signalwhile gradually changing the empty channel wavelength in the vicinity of1550 nm. And the reception node detects the crosstalk using this pseudoWDM signal. As a result, the crosstalk can be detected with goodaccuracy even when the zero-dispersion wavelength of the DSF hasvariations, so that a type of an optical fiber is determined withimproved accuracy.

FIG. 11 is a flowchart illustrating an exemplary method for determininga type of an optical fiber. Processing in this flowchart is executed bya pair of optical nodes. It is assumed in the following that theprocessing is executed by the optical nodes 1 a and 1 b illustrated inFIG. 2 .

In step S1, the controller 18 activates the optical amplifier circuit(EDFA) 12, the ASE light source 17, the WSS 11, the OTDR 13, and theoptical channel monitor (OCM) 15. This operation is performed in each ofthe optical nodes 1 a and 1 b.

In step S2, the OTDR 13 of the optical node 1 a measures a propagationtime between the optical node 1 a and a specified discontinuity, usingeach of a wavelength λ1 and a wavelength λ2. As a result, a propagationtime T1 and a propagation time T2 are respectively obtained for thewavelength λ1 and the wavelength λ2.

In step S3, the determination unit 18 a of the optical node 1 adetermines whether a difference ΔT between the propagation time T1 andthe propagation time T2 is larger than a specified threshold value TH1.When the difference ΔT is larger than the threshold value TH1, thedetermination unit 18 a determines that the optical fiber between theoptical nodes 1 a and 1 b is the SMF. On the other hand, when thedifference ΔT is smaller than the threshold value TH1, the determinationunit 18 a determines that the optical fiber between the optical nodes 1a and 1 b is a fiber different from the SMF (i.e., the DSF or theNZ-DSF).

When the difference ΔT is smaller than the threshold value TH1, in stepS4, the controller 18 of the optical node 1 a generates a pseudo WDMsignal. Specifically, the controller 18 gives to the WSS 11 aninstruction to generate a WDM signal including an empty channel. Theempty channel has the same or almost the same wavelength as thezero-dispersion wavelength of the DSF. The WSS 11 processes ASE light inaccordance with the instruction given by the controller 18, therebygenerating the pseudo WDM signal. Thereafter, the optical node 1 acauses the optical amplifier circuit 12 to amplify the pseudo WDMsignal, and transmits the pseudo WDM signal to the optical node 1 b.

In step S5, the optical node 1 b receives the pseudo WDM signal. In theoptical node 1 b, the optical channel monitor 15 measures power of eachwavelength channel and power of the empty channel in the pseudo WDMsignal. The controller 18 calculates crosstalk FWMXT occurring at theoptical fiber between the optical nodes 1 a and 1 b, based on a resultof the measurement by the optical channel monitor 15. Thereafter, theoptical node 1 b reports the calculated crosstalk FWMXT to the opticalnode 1 a.

In step S6, the determination unit 18 a of the optical node 1 adetermines a type of the optical fiber between the optical nodes 1 a and1 b, based on the crosstalk FWMXT reported from the optical node 1 b.Specifically, when the crosstalk FWMXT is larger than a specifiedthreshold value TH2, the determination unit 18 a determines that theoptical fiber between the optical nodes 1 a and 1 b is the DSF. On theother hand, when the crosstalk FWMXT is smaller than the specifiedthreshold value TH2, the determination unit 18 a determines that theoptical fiber between the optical nodes 1 a and 1 b is a fiber differentfrom the DSF (i.e., the SMF or the NZ-DSF). However, it is determined instep S3 that the optical fiber is different from the SMF. Therefore,when the crosstalk FWMXT is smaller than the specified threshold valueTH2, it is determined that the optical fiber is the NZ-DSF.

In the example illustrated in FIG. 11 , it is determined whether theoptical fiber is the SMF, and then it is determined whether the opticalfiber is the DSF; however, an embodiment of the present invention is notlimited to this procedure. That is, it may be determined whether theoptical fiber is the DSF, and then it may be determined whether theoptical fiber is the SMF.

In the foregoing example, the optical node 1 b detects the crosstalk,and the optical node 1 a determines the type of the optical fiber basedon the value of the crosstalk; however, an embodiment of the presentinvention is not limited to this procedure. For example, the opticalnode 1 b which has received the pseudo WDM signal may determine the typeof the optical fiber based on the crosstalk, and then may report aresult of the determination to the optical node 1 a.

According to an embodiment of the present invention, as described above,a pair of optical nodes cooperatively operates to determine a type of anoptical fiber between the optical nodes. Therefore, a type of an opticalfiber can be determined without a necessity that operators arepositioned at nodes on two ends of a target optical fiber, resulting inlabor and cost savings in a communication carrier. In many cases, anoptical node such as a ROADM includes an ASE light source, a WSS, and anoptical channel monitor. It is therefore unnecessary to add a componentor a circuit dedicated for transmitting and receiving a pseudo WDMsignal.

FIG. 12 illustrates another example of an optical node realized by aROADM. The optical node illustrated in FIG. 12 have substantially thesame configurations and operations as those of the optical node 1illustrated in FIG. 3 . It should be noted that FIG. 12 does notillustrate a controller 18.

FIG. 13 illustrates another exemplary optical transmission systemaccording to an embodiment of the present invention. The opticaltransmission system illustrated in FIG. 13 includes an optical node 3realized by an in-line amplifier (ILA) in addition to the optical nodes1 (1 a, 1 b) each realized by the ROADM illustrated in FIG. 2 . Theoptical node 3 is operable as an optical relay station. Specifically,the optical node 3 amplifies an optical signal transmitted from theoptical node 1 a, and transmits the amplified optical signal to theoptical node 1 b. Likewise, the optical node 3 amplifies an opticalsignal transmitted from the optical node 1 b, and transmits theamplified optical signal to the optical node 1 a.

FIG. 14 illustrates an exemplary optical node operable as an opticalrelay station. The optical node 3 operable as the optical relay stationincludes an optical amplifier circuit 32, an OTDR 33, an optical switch34, an optical channel monitor 35, an optical switch 36, and acontroller 38. The OTDR 33, the optical switch 34, the optical channelmonitor 35, and the optical switch 36 are substantially the same as theOTDR 13, the optical switch 14, the optical channel monitor 15, and theoptical switch 16 illustrated in FIG. 3 . It should be noted that theoptical node 3 does not need to include the ASE light source 17illustrated in FIG. 3 . In this case, the optical node 3 does notgenerate the pseudo WDM signal illustrated in FIG. 9B.

As illustrated in FIG. 14 , the optical amplifier circuit 32 includes adynamic gain equalizer (DGE) 39 in addition to an optical amplifier suchas an EDFA. The DGE 39 is capable of individually controlling power ofeach wavelength channel, in accordance with an instruction given by thecontroller 38. In this example, when the optical node 3 receives thepseudo WDM signal illustrated in FIG. 9B, the DGE 39 makes the power ofthe empty channel substantially zero. In other words, the DGE 39interrupts or blocks the empty channel.

For example, as illustrated in FIG. 13 , when the optical node 1 atransmits the pseudo WDM signal to the optical node 1 b, this pseudo WDMsignal is relayed by the optical node 3. At this time, crosstalk occursin an optical fiber RX1 between the optical node 1 a and the opticalnode 3. Therefore, the optical channel monitor 35 of the optical node 3measures power of the empty channel in the pseudo WDM signal (i.e.,crosstalk). It can thus be determined whether the optical fiber RX1 is aDSF based on a level of the crosstalk.

In the optical node 3, the DGE 39 makes the power of the empty channelin the pseudo WDM signal zero. The optical node 3 transmits to theoptical node 1 b the pseudo WDM signal for which the power of the emptychannel is made zero. In other words, a signal to be transmitted fromthe optical node 3 to the optical node 1 b is substantially the same asa pseudo WDM signal that has been transmitted from the optical node 1 ato the optical node 3. The optical node 1 b is therefore capable ofdetermining whether the optical fiber between the optical node 3 and theoptical node 1 b is the DSF.

Variation 1

In an optical transmission system 100B illustrated in FIG. 15 , twotypes of optical fibers are laid on a span between optical nodes 1 a and1 b. Specifically, an optical fiber 2 x through which an optical signalpropagates from the optical node 1 a to the optical node 1 b includes anoptical fiber F1 and an optical fiber F2. The same applies to an opticalfiber 2 y through which an optical signal propagates from the opticalnode 1 b to the optical node 1 a. In this case, when an OTDR 13implemented on the optical node 1 a or 1 b is used for measuring apropagation time, an average or total propagation time between the twooptical fibers is merely obtained. It is therefore difficult to specifythe types of the optical fibers. According to Variation 1, hence, thetypes of the optical fibers are specified using a front OTDR and a rearOTDR.

For example, the OTDR 13 implemented on the optical node 1 a emits anoptical pulse to the optical fiber F1, and measures the propagation timeusing reflected light from a discontinuity on the optical fiber.Likewise, the OTDR 13 implemented on the optical node 1 b emits anoptical pulse to the optical fiber F2, and measures the propagation timeusing reflected light from the discontinuity on the optical fiber. Aconnection between the optical fiber F1 and the optical fiber F2corresponds to a discontinuity on an optical fiber and causes Fresnelreflection. The optical node 1 a is therefore capable of determiningwhether the optical fiber F1 is an SMF. The optical node 1 b is alsocapable of determining whether the optical fiber F2 is an SMF.

In an optical transmission system 100C illustrated in FIGS. 16A and 16B,an optical fiber 2 x is laid, through which an optical signal propagatesfrom an optical node 1 a to an optical node 1 b. The optical fiber 2 xincludes an optical fiber F1 and an optical fiber F2. In the exampleillustrated in FIG. 16A, the optical node 1 a transmits a pseudo WDMsignal to the optical node 1 b, and an optical channel monitor of theoptical node 1 b detects crosstalk. In the example illustrated in FIG.16B, the optical node 1 b transmits a pseudo WDM signal to the opticalnode 1 a, and an optical channel monitor of the optical node la detectscrosstalk.

When the type of the optical fiber F1 is the same as the type of theoptical fiber F2 (or when one optical fiber is laid between the opticalnodes 1 a and 1 b), crosstalk to be detected in the optical node 1 a issubstantially the same as crosstalk to be detected in the optical node 1b. In the examples illustrated in FIGS. 16A and 16B, however, thecrosstalk to be detected in the optical node 1 a is different from thecrosstalk to be detected in the optical node 1 b. It is considered inthis case that the type of the optical fiber F is different from thetype of the optical fiber F2.

In an optical fiber, a level of crosstalk occurring by four-wave mixingdepends on the intensity of propagating light. Specifically, crosstalkoccurring in a region with high optical power is dominant.

In the case illustrated in FIG. 16A, a pseudo WDM signal generated bythe optical node 1 a is input to the optical fiber F1. Therefore, powerof a pseudo WDM signal propagating through the optical fiber F1 ishigher than power of a pseudo WDM signal propagating through the opticalfiber F2. In this case, a level of the crosstalk to be detected in theoptical node 1 b depends on the type of the optical fiber F1. In thisexample, the crosstalk to be detected in the optical node 1 b issufficiently large. In this case, it is therefore determined that theoptical fiber F1 is a DSF.

In the case illustrated in FIG. 16B, a pseudo WDM signal generated bythe optical node 1 b is input to the optical fiber F2. Therefore, thepower of the pseudo WDM signal propagating through the optical fiber F2is higher than the power of the pseudo WDM signal propagating throughthe optical fiber F1. In this case, a level of the crosstalk to bedetected in the optical node 1 a depends on the type of the opticalfiber F2. In this example, the crosstalk to be detected in the opticalnode 1 a is sufficiently small. In this case, it is therefore determinedthat the optical fiber F2 is different from the DSF.

It is determined whether each of the optical fibers F1 and F2 is an SMF,through the procedure illustrated in FIG. 15 , as described above.Therefore, a combination of the procedure illustrated in FIG. 15 withthe procedure illustrated in FIGS. 16A and 16B enables a determinationas to whether each of the optical fibers F1 and F2 is the SMF, the DSF,or an NZ-DSF.

FIG. 17 illustrates an exemplary configuration of one of the opticalnodes 1 used in the optical transmission system 100C illustrated inFIGS. 16A and 16B. In this case, the optical node 1 is required totransmit a pseudo WDM signal not only to an optical fiber fortransmitting an optical signal (an optical fiber TX in FIG. 17 ) butalso to an optical fiber for receiving an optical signal (an opticalfiber RX in FIG. 17 ). The optical node 1 therefore includes opticalswitches 41 and 42.

In inputting a pseudo WDM signal to the optical fiber TX, the opticalswitch 41 is controlled to guide a pseudo WDM signal generated by theWSS 11 to the optical fiber TX. In inputting a pseudo WDM signal to theoptical fiber RX, the optical switch 41 is controlled to guide thepseudo WDM signal generated by the WSS 11 to the optical switch 42, andthe optical switch 42 is controlled to guide the pseudo WDM signal tothe optical fiber RX. It should be noted that the optical switches 41and 42 are controlled by the controller 18.

Variation 2

As described above, each optical node 1 is capable of determining a typeof an optical fiber connected thereto. The communication carrierconfigures wavelength channels in a WDM signal in accordance with thetype of the optical fiber in order to improve communication quality.According to Variation 2, the optical node 1 makes this settingautomatically.

FIGS. 18A-18D illustrate an exemplary method for controllingtransmission power of a WDM signal in accordance with a type of anoptical fiber. It is assumed in this example that each optical node 1transmits a WDM signal. The controller 18 sets average transmissionpower of a plurality of wavelength channels constituting the WDM signaland a tilt compensation amount of the WDM signal, in accordance with atype of an optical fiber connected to the optical node 1. For example,the tilt compensation amount is set such that the spectrum of the WDMsignal becomes flat in a reception node.

The optical node 1 includes a parameter table illustrated in FIG. 18A.The parameter table stores a value of transmission power of eachwavelength channel for each optical fiber type. In this case, a value oftransmission power may be prepared for a combination of a type of anoptical fiber with a transmission distance. In a case where two types ofoptical fibers are mixed in one span, a value of transmission power maybe prepared for a combination of the two types of the optical fibers.

For example, in a case where the optical fiber connected to the opticalnode 1 is an SMF, as illustrated in FIG. 18B, a tilt is formed in eachsignal wavelength band (C-band, L-band) such that power of a channelwith a longer wavelength becomes smaller than power of a channel with ashorter wavelength. In the following description, a pattern of thetransmission power set for the SMF may be referred to as a “referencepattern”.

In a case where the optical fiber connected to the optical node 1 is aDSF, as illustrated in FIG. 18C, transmission power in the C-band is setto be smaller than the reference pattern, and transmission power in theL-band is set to be larger than the reference pattern. In a case wherethe optical fiber connected to the optical node 1 is an NZ-DSF, asillustrated in FIG. 18D, transmission power in the C-band is set to beequal to the reference pattern, and transmission power in the L-band isset to be smaller than the reference pattern.

The settings of transmission power may be realized by the WSS 11 and theoptical amplifier circuit 12. Specifically, the transmission power iscontrolled by the controller 18 to set parameters for the WSS 11 and theoptical amplifier circuit 12. In this case, the controller 18 setsparameters for controlling the power of each wavelength channel, for theWSS 11. The tilt of the WDM signal is formed by this setting. Inaddition, the controller 18 controls a gain of the optical amplifiercircuit 12. The average power of the WDM signal is thus controlled.

Variation 3

As described above, in determining a type of an optical fiber using apseudo WDM signal, a pair of optical nodes 1 implemented on two ends ofthe target optical fiber cooperatively operates. At this time, eachoptical node 1 transmits a control signal using an optical supervisorychannel (OSC).

FIG. 19 illustrates an exemplary sequence of cooperative operation by apair of optical nodes. In this example, the optical node la determines atype of the optical fiber between the optical nodes 1 a and 1 b.

First, the optical node 1 a transmits to the optical node 1 b aninstruction to start measurement of crosstalk. This start instruction istransmitted using the OSC. As illustrated in FIG. 20 , for example, theOSC is set outside a signal band for transmitting a WDM signal. Whenreceiving the start instruction, the optical node 1 b prepares forreception of a pseudo WDM signal. Thereafter, the optical node 1 btransmits a response signal to the optical node 1 a using the OSC.

When receiving the response signal, the optical node 1 a transmits apseudo WDM signal to the optical node 1 b. The pseudo WDM signal isgenerated by the ASE light source 17 and the WSS 11, based on aninstruction from the controller 18, as described above. The optical node1 b measures crosstalk caused by four-wave mixing using the pseudo WDMsignal. The optical node 1 b then reports a result of the measurement ofthe crosstalk to the optical node 1 a, using the OSC.

The optical node la determines a type of the optical fiber between theoptical nodes 1 a and 1 b, based on the measurement result of thecrosstalk reported from the optical node 1 b. The result of thedetermination is displayed on, for example, a computer of thecommunication carrier. Thereafter, the optical node 1 a may setparameters for the optical transmission circuit (such as the WSS 11 andthe optical amplifier circuit 12) of the optical node 1 a in accordancewith the type of the optical fiber.

In the example illustrated in FIG. 19 , the optical node 1 b reports theresult of the measurement of the crosstalk in optical node 1 a, and theoptical node 1 a determines the type of the optical fiber, based on theresult of the measurement of the crosstalk; however, an embodiment ofthe present invention is not limited to this sequence. For example, theoptical node 1 b may measure the crosstalk and then determine the typeof the optical fiber based on a result of the measurement. In this case,the optical node 1 b reports a result of the determination as to thetype of the optical fiber to the optical node 1 a, using the OSC.

FIG. 21 illustrates an exemplary optical node including an OSCprocessor. An optical node 1 illustrated in FIG. 21 includes an OSCprocessor 51 in addition to the configurations illustrated in FIG. 3 .The OSC processor 51 transmits an optical signal indicating a controlsignal generated by a controller 18. In the example illustrated in FIG.19 , the OSC processor 51 transmits an OSC optical signal indicating astart instruction. In addition, the OSC processor 51 receives an OSCoptical signal transmitted from another node, extracts a control signalfrom this OSC optical signal, and sends the control signal to thecontroller 18. In the example illustrated in FIG. 19 , the OSC processor51 receives a response and a result of measurement of crosstalk from theoptical node 1 b.

Variation 4

In the examples illustrated in FIGS. 8-10 , it is determined whether atarget optical fiber is a DSF, by measuring crosstalk using a pseudo WDMsignal. According to Variation 4, in contrast, it is determined whethera target optical fiber is an SMF, a DSF, or an NZ-DSF, by changing aconfiguration of a pseudo WDM signal.

FIG. 22 is a flowchart illustrating an exemplary method for identifyingan SMF, a DSF, and an NZ-DSF, using a pseudo WDM signal. It should benoted that steps S11-S13 are substantially the same as steps S4-S6 inthe flowchart illustrated in FIG. 11 . Specifically, the controller 18makes the ASE light source 17 and the WSS 11 to generate a pseudo WDMsignal including an empty channel near the zero-dispersion wavelength ofthe DSF. This pseudo WDM signal is transmitted to an adjacent nodethrough the target optical fiber. In the adjacent node, crosstalk iscalculated by measuring power of the empty channel. As described abovewith reference to FIGS. 9A-9D, it is determined that the target opticalfiber is the DSF when this crosstalk is larger than a specifiedthreshold value.

In step S14, the controller 18 makes the ASE light source 17 and the WSS11 to generate a pseudo WDM signal including an empty channel near thezero-dispersion wavelength of the NZ-DSF. In this example, asillustrated in FIG. 23A, the zero-dispersion wavelength of the NZ-DSF isapproximately 1500 nm, and the wavelength of the empty channel is 1525nm. The pseudo WDM signal including this empty channel is transmitted tothe adjacent node through the target optical fiber.

In step S15, the controller 18 of the adjacent node makes the opticalchannel monitor 15 to measure power of the empty channel, therebycalculating crosstalk caused by four-wave mixing. In step S16, thedetermination unit 18 a of the optical node 1 or the determination unit18 a of the adjacent node compares the crosstalk obtained in step S15with a specified threshold value. This threshold value is preferably setindependently of the threshold value used in step S13. It is determinedthat the target optical fiber is the NZ-DSF when the crosstalk obtainedin step S15 is larger than the threshold value as illustrated in FIG.23B. On the other hand, it is determined that the target optical fiberis the SMF when the crosstalk obtained in step S15 is smaller than thethreshold value as illustrated in FIG. 23C. According to Variation 4, asdescribed above, an optical fiber provided between optical nodes can beidentified as an SMF, a DSF, or an NZ-DSF, without use of an OTDR.

All examples and conditional language provided herein are intended forthe pedagogical purposes of aiding the reader in understanding theinvention and the concepts contributed by the inventor to further theart, and are not to be construed as limitations to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although one or more embodiments of thepresent inventions have been described in detail, it should beunderstood that the various changes, substitutions, and alterationscould be made hereto without departing from the spirit and scope of theinvention.

What is claimed is:
 1. An optical transmission system that includes afirst optical node, a second optical node, and an optical fiber providedbetween the first optical node and the second optical node, the opticaltransmission system comprising: a signal generator provided in the firstoptical node and configured to generate an optical signal including aplurality of wavelength channels and an empty channel; an opticaltransmission circuit provided in the first optical node and configuredto output the optical signal to the optical fiber; an optical channelmonitor provided in the second optical node and configured to measurereception power of each channel in the optical signal received throughthe optical fiber; and a processor configured to determine a type of theoptical fiber based on the reception power of the empty channel, thereception power being measured by the optical channel monitor.
 2. Theoptical transmission system according to claim 1, wherein the emptychannel is provided in a zero-dispersion wavelength of a dispersionshifted single-mode optical fiber or near the zero-dispersion wavelengthof the dispersion shifted single-mode optical fiber, and the processordetermines whether the optical fiber is the dispersion shiftedsingle-mode optical fiber based on the reception power of the emptychannel.
 3. The optical transmission system according to claim 1,wherein the empty channel is provided in a zero-dispersion wavelength ofa non-zero dispersion shifted single-mode optical fiber or near thezero-dispersion wavelength of the non-zero dispersion shiftedsingle-mode optical fiber, and the processor determines whether theoptical fiber is the non-zero dispersion shifted single-mode opticalfiber based on the reception power of the empty channel.
 4. The opticaltransmission system according to claim 1, wherein the processorcalculates crosstalk in a wavelength in which the empty channel isprovided, the crosstalk occurring due to the plurality of wavelengthchannels in the optical fiber, based on a ratio between the receptionpowers of the plurality of wavelength channels and the reception powerof the empty channel, and the processor determines the type of theoptical fiber based on a level of the crosstalk.
 5. The opticaltransmission system according to claim 1, wherein the signal generatorincludes: an ASE light source configured to generate ASE light; and awavelength selective switch configured to generate the optical signalfrom the ASE light.
 6. The optical transmission system according toclaim 1, wherein the processor is provided in the first optical node,the second optical node reports a result of measurement by the opticalchannel monitor to the first optical node, and the processor determinesthe type of the optical fiber based on the result of the measurementreported from the second optical node.
 7. The optical transmissionsystem according to claim 1, wherein the first optical node furtherincludes an optical measuring device configured to emit to the opticalfiber a first optical pulse in a first wavelength and a second opticalpulse in a second wavelength and to detect reflected light of the firstoptical pulse from a discontinuity on the optical fiber and reflectedlight of the second optical pulse from the discontinuity on the opticalfiber, and the processor determines whether the optical fiber is asingle-mode optical fiber based on a difference between a firstpropagation time and a second propagation time, the first propagationtime indicating a period from a transmission of the first optical pulseto a detection of the reflected light of the first optical pulse fromthe discontinuity and the second propagation time indicating a periodfrom a transmission of the second optical pulse to a detection of thereflected light of the second optical pulse from the discontinuity. 8.The optical transmission system according to claim 1, further comprisingan optical relay station provided between the first optical node and thesecond optical node, wherein the optical relay station blocks the emptychannel in relaying the optical signal to the second optical node. 9.The optical transmission system according to claim 1, wherein the secondoptical node includes: a second signal generator configured to generatea second optical signal including a plurality of wavelength channels andan empty channel; and a second optical transmission circuit configuredto output the second optical signal to the optical fiber, the firstoptical node includes a second optical channel monitor configured tomeasure reception power of each channel in the second optical signalreceived through the optical fiber, and the processor determines thetype of the optical fiber based on the reception power of the emptychannel in the optical signal measured by the optical channel monitor,and the reception power of the empty channel in the second opticalsignal measured by the second optical channel monitor.
 10. The opticaltransmission system according to claim 9, wherein when the receptionpower of the empty channel in the optical signal measured by the opticalchannel monitor is different from the reception power of the emptychannel in the second optical signal measured by the second opticalchannel monitor, the processor determines that the optical fiberincludes a first optical fiber and a second optical fiber that isdifferent in type from the first optical fiber, and determines a type ofthe first optical fiber and a type of the second optical fiber based onthe reception power of the empty channel in the optical signal measuredby the optical channel monitor and the reception power of the emptychannel in the second optical signal measured by the second opticalchannel monitor.
 11. The optical transmission system according to claim1, wherein the processor is provided in the first optical node andcontrols transmission power of each wavelength channel in a wavelengthdivision multiplexing signal to be transmitted to the second opticalnode in accordance with the determined type of the optical fiber.
 12. Afiber type determination method for determining a type of an opticalfiber provided between a first optical node and a second optical node,the fiber type determination method comprising: transmitting an opticalsignal including a plurality of wavelength channels and an empty channelfrom the first optical node to the second optical node through theoptical fiber; measuring, in the second optical node, reception power ofeach channel in the optical signal received through the optical fiber;and determining a type of the optical fiber based on the reception powerof the empty channel, the reception power being measured in the secondoptical node.